MINOR: glitches: add global setting "tune.glitches.kill.cpu-usage"

MINOR: glitches: add global setting "tune.glitches.kill.cpu-usage"

It was mentioned during the development of glitches that it would be
nice to support not killing misbehaving connections below a certain
CPU usage so that poor implementations that routinely misbehave without
impact are not killed. This is now possible by setting a CPU usage
threshold under which we don't kill them via this parameter. It defaults
to zero so that we continue to kill them by default.